The present invention relates to a method and apparatus for determining a chemical component from a sample of matter, in particular for determining the glucose content of blood from a blood sample.
It is generally known that diabetics are treating themselves in daily life. This is made possible by the use of domestic blood glucose measurement. In the known methods, patients with diabetes place a drop of blood on a test strip, which contains the reagent. The reagent will react with the glucose content of the blood, and generates a well-defined color. The reaction is a multi-stage reaction and is commonly known. The glucose-oxidase enzyme creates hydrogen-peroxide (H2O2) from the glucose content of the blood, the oxygen of the air and of the water present in the blood. The amount of the H2O2 generated is proportional to the amount of the glucose, and a peroxidase enzyme further activates it. The activated H2O2 oxidizes the indicator (also commonly known) in the test strip, which will change its color. This change of color may be accurately measured.
Earlier test strips have been washed or wiped after the application of the sample, and the color has been determined by comparison with a color chart. More recently, the color has been determined by a small electronic reading device, which calculated automatically the glucose content of the blood sample. The modem test strips are of the so-called no-wipe type, i.e., the blood sample need not be wiped or washed off. With these no-wipe strips the detection of the color reaction is performed on the opposite side to where the sample has been placed. The test strip is provided with a reagent carrier, usually a textile or foil patch, and the test strip is provided with a hole, through which the opposite side of the reagent carrier may be observed. These test strips are almost exclusively analyzed by reading devices, which provide much more objective measurement than the subjective comparison with the color chart. During the reading, the previous devices have measured the reflection of the reagent carrier on a predetermined wavelength. The color generated by the color reaction in the reflection carrier, or more properly on the back side of the carrier, is deduced from the measured reflection value.
The color reaction on such test strips progresses relatively fast, and both at the start of the reaction and after the completion of the reaction, various effects can occur, which affect the results of the color reaction itself. Therefore, in order to determine precisely the result of the color reaction, it is important to perform the reflection measurement serving as the basis of the glucose measurement in a well-defined time interval. Only in this manner is it possible to calibrate properly the relation between the color and the sugar content of the sample.
With the first known devices the measurements were done in the following manner: The patient switched on the device or switched from the stand-by state into the measuring state, after having positioned the sample. This method was not adequate, because the delays until the measurement actually started were varying, due to the switching on, even with by same person. Therefore the need arose to develop such methods, which ensured that the interval between placing the sample on the test strip and the start of the measurements could be determined uniformly, so that the precision of the measurements could improve. Therefore it is desirable to detect automatically, with the measurement device, the start of the color reaction and to detect its shape, so that the device could automatically determine a following time interval when the reflection measurement should be performed. The reflection measurement made in this time interval then could serve as a basis for the determination of the glucose content. Alternatively, in the case of continuous or sampled measurement it is sought to determine when the time Tm (time point) occurs, so that a single reflection value measured in this time point Tm could be the basis of the determination of the glucose content. The general object of the present invention is to provide a method for determining this Tm time point.
When determining this time point, several factors must be considered, which may present contradictory demands. Of course, it is of primary concern that the Tm time point of the measurement should be determined in a reproducible manner, as well as the R reflection values measured in the Tm time points so determined. The deduced blood glucose values should also be reproducible, i.e., the accuracy of the blood glucose measurements must not be worse than with known methods.
On the other hand, it is desirable to perform the measurement as quickly as possible, which is, firstly, convenient for the patient, and, secondly, so the battery in the measurement device may last longer. On the other hand, laboratory measurements have shown that the ideal time point for the measurement is dependent on the glucose content of the sample itself. With certain types of test strips it is advantageous to measure earlier the samples with lower glucose content, than those with a higher glucose content. The reason for this is that with some test strips the color reaction takes longer with higher glucose content. Conversely, there are test strips where the opposite is true, that is samples with higher glucose content should be measured earlier, because the color reaction is faster with the higher glucose content and the result is reached earlier. It is advisable to wait longer with low glucose samples until the end of the reaction or close to the end, in order to be able to determine the glucose content precisely. In other words, a good system must be capable of xe2x80x9crecognizingxe2x80x9d, even before the final measurement, what the interval should be, and the measurement time Tm can be adjusted accordingly.
This is achieved in newer devices by measuring quasi-continuously the reflection curve, and by determining dynamically the Final measurement time. This latter process contradicts the requirement for simple operations and calculations. This is an important aspect, because the blood glucose measurement devices should be small and portable (i.e., operating from battery), be simple to operate, and, last but not least, be cheap.
A continuous reflection measurement requires the continuous or frequent switching on/off of the light source, typically a LED, and inevitably have a high power consumption. Therefore, it is sought to substitute the continuous measurement with sampling on a frequency as low as possible. It may also be mentioned that a more complicated method requires a more sophisticated controlling processor, which is more expensive. On the other hand, a more complicated algorithm, in a given processor in a given time, allows the evaluation of fewer measurement points, which in turn will result in a less precise measurement. It is less significant, but may be taken into consideration that the power consumption of the processor is higher with more calculating steps. This latter factor may play a role if the controlling algorithm of the device is not made by digital processor but by analog circuits, e.g. due to considerations of reliability. With higher power consumption the device will operate for a shorter time, so indirectly its reliability will worsen (i.e. the probability of malfunction due to the run-down of the batteries will increase).
The document U.S. Pat. No. 4,199,261 (Tidd et al.) discloses an optical reflection meter, which is used to determine the glucose content in urine of diabetics. The device is capable of determining if the sample carrier is dry or wet, by comparing the measured reflection with a predetermined threshold value. The value measured on the dry sample carrier is used for calibrating the device. Following this, the user inserts the sample carrier, which has been wetted with the urine sample, in the device, which is automatically identified by the device. After this, the final measurement is made after a predetermined time interval following the recognition of the wet sample.
The documents U.S. Pat. Nos. 4,935,346 and 5,049,487 (Phillips et al) disclose a method similar to the previous method, but primarily for determining the sugar content of a blood sample. The device to perform the method is described in the document U.S. Pat. No. 5,059,394. This known method differs from the previous one in that placement of the blood samplexe2x80x94in practice, applying a drop of blood on the sample carrier, in this case a no-wipe test stripxe2x80x94causes the decrease of the reflection, which is detected immediately by the device. Thus the short, but uncertain time interval is excluded, which will necessarily arise in the previously described method of the document U.S. Pat. No. 4,199,261 (Tidd et al.), because of the delay between the wetting of the sample carrier with the urine and placing the sample carrier in the device.
In the method described in the document U.S. Pat. No. 4,935,346, the blood sample penetrates the sample carrier, which serves simultaneously as the reagent carrier, and the effective measurement is performed after a predetermined time, following the detection of the decrease in the reflection. This method effectively excludes the subjective elements of the measurement, but its disadvantage is that the measurement time is determined independently of the glucose content. It is a further disadvantage that it needs frequent samplings, to determine the exact time of wetting through of the sample, because the reflection curve is failing very steeply around the critical time. If sampling is made at longer intervals, the determination of the T0 starting time will be less exact, and from there it follows that the time of the final measurement will also fluctuate in relation to the ideal measurement time determined by the calibration curve. E.g. with higher glucose contents, if the reaction is still in progress in the predetermined measurement time, the uncertainty of the measurement time will be reflected in the measured results.
This latter method has been improved by Phillips et al. according to a method disclosed in the document U.S. Pat. No. 5,179,005. In this known method, based on the theoretical background of the so-called Kubelka-Munk equations, which are well known in the art, the so-called K/S values are calculated, and the blood glucose content is determined on the basis of these K/S values. The final measurement time which serves as the basis of the calculations, is still determined using a predetermined time interval following an initial decrease in the reflection. A disadvantage of this known method is that it is still not able to consider the order of magnitude of the measured glucose content when determining the measurement time. Thus, the measurement is not always made at the ideal time, and further, there must be a trade-off between the accuracy of the measurement and the sampling frequency.
Therefore, it is an object of the present invention to provide a method, which allows the determination of the measurement time in a manner avoiding or at least minimizing the disadvantages of the known solutions. Further objectives of the present invention are to determine the measurement time with a simple algorithm, and to determine a measurement time, which is set at or near the ideal time, dependent on the glucose content to be measured. It is still a further object to provide a method where the sampling frequency may be kept relatively low, in order to keep the power consumption low. Because the light sources of the measurement devices are normally the largest energy users, this is an important factor. The blood glucose measurements are also negatively affected by temperature variations, hence it is preferable that the method of the invention should deliver results independent of the measurement temperature.
In the method according to the present invention, the sample to be measured is positioned on one side of a test strip containing a reagent causing a color reaction directly or through an intermediate reaction with the clinical component to be measured. The components of the sample penetrate the test strip and start the color reaction at the other side of the test strip. The content of the component in the samplexe2x80x94in particular the glucose content of the blood samplexe2x80x94is determined by measuring through optical reflection measurement the result of the color reaction, particularly the developing color or darkening, and by comparing with earlier calibrating measurements. In the following, by color reaction any clinical or physicochemical reaction is meant that causes any change in the sample that may be indicated or measured by an optical reflection measurement. That is, the expression xe2x80x9ccolor reactionxe2x80x9d also includes any chemical reaction, where there is no real change of colorxe2x80x94the change of the spectrum of the reflected lightxe2x80x94but only the measured intensity is changed, that is some darkening or lightening is detected. Obviously, the above effects may appear mixed.
The invention further relates to an apparatus for determining a chemical component from a sample of matter, in particular for determining the glucose content of a blood sample, particularly for implementing the method according to the invention. The apparatus of the invention includes a sample holder accommodating the test strip which contains the chemistry for the reaction. The apparatus further includes a light source for illuminating the reaction area of the test strip in the holder, such as a light emitting diode (LED), and a circuit for measuring the intensity of the light reflected from the sample, such as a photo-detector. The functioning of the apparatus is controlled by a programmable controller and analyzer circuit, such as a microprocessor, for processing the signals of the circuit for measuring the light intensity and for determining the chemical component, such as the glucose content of the blood sample.
According to the present invention, the above objectives are realized by a method, whereby the sample is positioned on one side of a test strip, which contains a reagent causing a color reaction, directly or through an intermediate reaction with the chemical component to be measured. The components of the sample penetrate the test strip and start the color reaction at the other side of the test strip. The content of the component in the sample, such as the glucose content of the blood sample, is determined by measuring through optical reflection measurement the result of the color reaction, such as the developing color or darkening, and by comparing it with earlier calibrating measurements. According to one embodiment of the present invention, the method includes illuminating the sample and measuring at discrete time intervals or substantially continuously the R reflection on the test strip and recording the R(t) function. The method also recites the detection of the wetting through of the sample, and determines the T0 starting time, T0, the starting time being not earlier than the time of detection of wetting through. The method then calls for generating from the T0 starting time the function R(t)+L(t) where L(t) is a predetermined function, independent of the measured reflection, monitoring and storing the ext[R(t)+L(t)] extreme value of the R(t)4+L(t) functionxe2x80x94preferably its min[R(t)+L(t)] minimum value and, at the same time, generating the function R(t)+L(t)xe2x88x92ext[R(t)+L(t)] from the time of reaching at least one definite (true) ext[R(t)+L(t)] extreme value. When the R(t)+L(t)xe2x88x92ext[R(t)+L(t)] function reaches a predetermined C(t) value, the method calls for determining the Tm measuring time, and determining from the R reflection value measured at the Tm measuring time the content of the chemical component in the sample, preferably the glucose content of the blood sample.
The method according to the present invention is based on the recognition that the sudden reduction in reflection should not be the determining factor, but by utilizing the characteristic curve of the reaction being measured, that section must be found where the color reaction has already come to an end, but where the distorting effect of other phenomena has not yet affected or has only slightly affected the measurement result. We have discovered that at various glucose contents the color reaction progresses roughly at the same reaction rate. Though this color reaction does not fully separate in time from the wetting process, however, the manufacturers of test strips basically strive for this. They have already reached a point where, the sections with the maximum reaction rates of the characteristic reactions are separated. The color reaction involves a characteristic reaction rate, to which a predetermined slope of the reflection curve belongs. Therefore, the appropriate section of the reaction curve must be found which has a predetermined slope, and it can be conveniently found using the algorithm according to the invention. It will be apparent to one of ordinary skill in the art that the algorithm is easily programmable and that the results can be calculated with a simple, low-performance processor in just a few operational steps.
In certain cases, the L(t) value may be presented in tabular form and in this case the processor should perform only subtraction, addition and comparison, instead of division or multiplication. It can be recognized that the To starting time need not be specified exactly, as the aim is merely to ensure that the method should not begin the generation of the Rcorr=[R(t)+L(t)] value, or at least the search for the extreme value, until after the lapse of a predetermined period following the commencement of the wetting.
It is not necessary to determine the starting time of the wetting very precisely, since the method adjusts the final measurement to a time when the color reaction has already slowed down. That is, when the reflection curve changes more slowly, therefore, during the search for the minimum value the sampling frequency may be relatively low. However, attributable to the rather slow change in reflection, at the same time the exact determination of the final Tm time is less critical than in the already known methods.
On the other hand, when the actual values are determined by interpolating the measured values, then the method according to the present invention may be applied advantageously for the so-called quick strips, where the reactions take place quickly. For example, the sampling frequency can be reduced so that the Tm time point is made equal not to the n-th value Tn in which Tn time we would first observe the reaching of the C(t) function, but the exact value of Tm can be a point of time determined by interpolating between the time Tn and Tnxe2x88x921, where Tm is defined by the F*(Tm)=C value. Here, the function F*(t) is the linear or higher order approximation of the function F(t)=R(t)+L(t)xe2x88x92ext[R(t)xe2x88x92L(t)] laid through points Tnxe2x88x921, F(Tnxe2x88x921) and Tn, F(Tn). Of course, the measured reflection value Rm can also be calculated from the reflection values R(Tn) and R(Tnxe2x88x921) by interpolation. The error caused by the interpolation will be very small because in this time interval the variation of R is quite low. A further benefit is that the procedure is less sensitive to the deviations caused by the fluctuation of the reaction rate, because it adjusts the measurement according to the variation in the reaction rate. Therefore, the aging of the test strips, the measurement temperature, vapor content and other factors affecting the reaction rate will but slightly deteriorate the accuracy of the glucose content measurement.
It is also known that the test strip manufacturers are characterizing the different production batches with a so-called code. A code number identifies the characteristics of the test strips of a batch. In order to comply with increasingly exact measurement methods, the manufacturers are using a steadily growing number of the codes. The method according to the invention allows for the adjustment to the fine differences in the characteristics of test strips having many code numbers.
In one exemplary embodiment of the method according to the present invention, L(t) is a linear function with a predetermined slope [L(t)=At+B, preferably L(t)=At and B=O], while C(t) is a constant function [C(t)=C]. However, it is also possible that L(t) is a second-order or a higher order function of time. C(t) can be specified, for example, in a more general polynomial form as well. Preferably, the wetting through of the sample is detected on the basis of a predetermined amount of change in reflection. This requires merely a comparison with a threshold value, therefore, its demand for processing power is rather modest. However, it may be more advantageous if the wetting through of the sample is detected by a predetermined rate of change in reflection. With this method it is possible to filter out the effects of, for example, the inadvertent moving of the sample and the reductions in reflection not involving a real reaction. Alternatively, the wetting of the sample may be detected on the basis of the reflection value reaching a predetermined limit value. This simplifies the programming of the processor, because in the starting phase it requires the storing of a single reflection value at one time.
In one particular preferred embodiment, a blood sample, full blood, blood plasma or serum is tested using the present invention. In practice it worked well if the illumination was made with an intensity of 0.01 to 1 mW and with a wavelength of 400 to 1500 nm. The intensity and the wavelength of the illumination must be chosen so that the illumination will not affect the progress of the color reaction through an eventual photochemical reaction.
As noted above, the invention further relates to an apparatus for determining a chemical component from a sample, in particular for determining the glucose content of a blood sample, particularly for implementing the method according to the invention. The apparatus is composed of a sample holder accommodating the test strip applied in the method and entering into chemical reaction with the sample. The apparatus is further composed of a light source for illuminating the sample placed into the sample holder, such as a light emitting diode (LED), a circuit for measuring the intensity of the light reflected from the sample, such as a photo-detector, and includes a programmable controller and analyzer circuit, such as a microprocessor for processing the signals of the circuit for measuring the light intensity and for determining the chemical component, such as the glucose content of the blood sample. According to this embodiment of the present invention, the programmable controller and analyzer circuit, such as microprocessor 13, is programmed for the execution of an embodiment of the method according to the present invention.
Another embodiment of the present invention further relates to a microprocessor readable storage medium with executable instructions of a program for a microprocessor for determining a chemical component from a sample, such as determining the glucose content of blood from a blood sample. The storage medium according to this embodiment of the present inventions stores instructions for performing the following steps:
a) measuring at discrete time intervals or substantially continuously the R reflection of the illuminated sample on the test strip and recording the R(t) function;
b) detecting the wetting through of the sample;
c) at the detection of the wetting through of the sample, determining the T0 starting time, where T0 starting time is not earlier than the time of detection of wetting through;
d) generating from T0 starting time the function R(t)+L(t) in which L(t) is a predetermined function, independent of the measured reaction;
e) monitoring and storing the ext[R(t)+L(t)] extreme value of the R(t)+L(t) function, such as its minimum value min[R(t)+L(t)];
f) generating the function R(t)+L(t)xe2x88x92ext[R(t)+L(t)] from the time of reaching at least one definite (true) ext[R(t)+L(t)] extreme value;
g) when the R(t)+L(t)xe2x88x92ext[R(t)+L(t)] function reaches a predetermined C(t) value, determining the T0 measuring time; and
h) determining from the R reflection value measured at the Tm measuring time the content of the chemical component in the sample, preferably the glucose content of the blood sample.